The most commonly used accent in Italian is the grave (`), which can safely be used wherever a written accent is required. But there is a convention (by no means consistently followed) that an accent is written acute (´) on the vowel [e], and grave on [open e], which is why né, sé, perché are usually written with an acute, but è with a grave.

So then based on this my question is:

Earlier when ALBatro was present he corrected perché to perchè, so obviously there is a difference in pronounciation. But which perche is for 'why' and which is for 'because'?

In Italian the only two vowels which needs to have the right accent (because of different pronunciations) are e (è and é) and o (ò and ó). The other vowels have just one pronunciations and so the accent written on them has not any influence on the pronunciation. Anyway the right accents should be à, í and ú, although Italian keyboards have just the key à, ì and ù.
To end this post, I would like to say that "perchè" is never correct and the right word is "perché" for both "why" and "because".

After searching "accent" - to see if there is a difference between è and é, I came across this thread.

Annamaria wrote:
Earlier when ALBatro was present he corrected perché to perchè, so obviously there is a difference in pronounciation. But which perche is for 'why' and which is for 'because'?
Annamaria

So what's the answer? I am wondering this myself.

è and é must have more significance than just pronounciation. I base this upon: perche could mean why or because depending on the direction of the accent.